General Design Methodology for Organic Eutectic Electrolytes toward High‐Energy‐Density Redox Flow Batteries

Abstract

By virtue of strong molecular interactions, eutectic electrolytes provide highly concentrated redox‐active materials without other auxiliary solvents, hence achieving high volumetric capacities and energy density for redox flow batteries (RFBs). However, it is critical to unveil the underlying mechanism in this system, which will be undoubtedly beneficial for their future research on high‐energy storage systems. Herein, a general formation mechanism of organic eutectic electrolytes (OEEs) is developed, and it is found that molecules with specific functional groups such as carbonyl (CO), nitroxyl radical (NO•), and methoxy (OCH3) groups can coordinate with alkali metal fluorinated sulfonylimide salts (especially for bis(trifluoromethanesulfonyl)imide, TFSI), thereby forming OEEs. Molecular designs further demonstrate that the redox‐inactive methoxy group functionalized ferrocene derivative maintains the liquid OEE at both reduced and oxidized states. Over threefold increase in solubility is obtained (2.8 m for ferrocene derivative OEE) and high actual discharge energy density of 188 Wh L‐1 (75% of the theoretical value) is achieved in the Li hybrid cell. The established mechanism presents new ways of designing desirable electrolytes through molecular interactions for the development of high‐energy‐density organic RFBs.

Eutectic Electrolytes Chemistry for Rechargeable Zn Batteries

“…[44] Organic-based eutectic electrolytes, are a class of the facile eutectic system to boost the concentration of metal and organic active components given interactions effects, and additionally, an excellent exhibition of low vapor pressure and thermosta-bility heighten practical implementation. [89] Representatively, the low-cost DES using urea and ChCl with a stoichiometric ratio of 2:1 shows good solubility for various Zn salts [e.g., Zn sulfate (ZnSO 4 ), ZnCl 2 , Zn nitrate (Zn(NO 3 ) 2 ), and Zn acetate (Zn(Ac) 2 )], high ionic conductivity, and a wide electrochemical window (Figure 5c). The resulting eutectic electrolytes optimize the solvation structures of Zn ions in the electrolyte and allow dendrite-free Zn stripping/plating over 3920 h for Zn-S batteries.…”

“…Notably, we believe the versatile types and functionalities of eutectic electrolytes efficiently bridge the accelerated development of other energy storage devices (i.e., sodium, calcium, aluminum, and magnesium-based batteries/SCs) in the future. [89,103]…”

Strategies to Improve the Energy Density of Non-Aqueous Organic Redox Flow Batteries

“…The diversity in molecular structure and elemental abundance of organic molecules provide great opportunities for designing high-performance OEEs [40][41][42][43][44][45][46][47] . By mixing EG, malonic acid, and viologen-based ammonium salt, Goeltz et al 42 obtained a metalfree low melting point OEE with viologen concentration as high as 4.2 mol•L −1 .…”

“…Recently Yu and coworkers systematically studied the formation of OEEs between organic molecules with carbonyl/nitroxyl radical/methoxyl groups and alkali metal fluorinated sulfonylimide salts and proposed a general design methodology to prepare redox-active OEEs for RFBs (Fig. 6) 44 . Following this methodology, they found that methoxysubstituted Fc derivative remains liquid at both reduced and oxidized states.…”

Eutectic Electrolytes in Advanced Metal-Ion Batteries

“…It can be seen that IL electrolytes are not economical and friendly, 28 aqueous electrolytes have a limited electrochemical window, 29 and organic electrolytes are usually toxic and cause environmental damage. 30,31 In contrast, eutectic electrolytes have a wide electrochemical window and superior thermal/chemical stability as well as being nontoxic and inexpensive. Meanwhile, due to the presence of complex anions and cations, the eutectic electrolytes also possess characteristics that the other electrolytes do not have, including non-flammability, easy synthesis, structural flexibility, and multi-electron reactions (Figure 1d).…”